Converting positive earth clock to negative earth

[Mr Spigot]

In my garage clearout, I came across a new old stock Smiths 12v transistor clock (CTE 3103/00) - see photos. The reason I never fitted it was because it is positive earth, whereas my car is negative earth. Is there any way it can be easily adapted to work with a negative earth circuit? I thought of reversing the connections, but that is too obvious and would probably damage it. If not, I will have to pass it on to someone who can use it.

[geoberni]

Looking at that, there's a Red cable for an inbuilt lamp, and the Yellow powers the clock.
You'll be OK so long as the case is insulated from the Car Earth, i.e. the body or metal mounting bracket.
So mount it in an all Plastic Gauge Holder bracket and you can reverse the connections.

[Mr Spigot]

Brilliant! That is just what I needed to know. Thanks for your help.

So just to confirm, the red wire connects to the lighting circuit, the yellow to earth and the 'earth' connector to a live feed?

[StillGotMy1stCar]

Nice clock.
May be worth checking the clock works before you go to the trouble of working out how to mount it.
If you isolate the case and wire it with the work around described above by Berni, don’t connect the red illumination wire as it will feedback through the car lighting circuit, i.e., the clock will illuminate continuously draining the battery when the car lights are off and when the car lights are on the clock light will be off.

Regards John

[geoberni]

... don’t connect the red illumination wire as it will feedback through the car lighting circuit, i.e., the clock will illuminate continuously draining the battery when the car lights are off and when the car lights are on the clock light will be off.

Regards John

yes, I hadn't thought of that, well spotted.
As the Clock would be powered all the time, and the Positive would be on the lamp case, it would require a separate switch to earth the clock lighting.
I would suggest a push button or momentary action switch that you could push just to illuminate the clock when you want to see it at night

I've looked up that clock number Smiths CTE 3103/00... and found a few interesting snippets. Not only is there quite a market in them, one listing on ebay seems to have gone for £80, but also there are repair kits available for them.

See this page https://www.clocks4classics.com/uploads ... cks_v1.pdf
It mentions there on p16 that some Smiths Clocks had a Capacitor...

Capacitor.JPG (77.95 KiB) Viewed 1226 times

.
That looks to me as if it is a electrolytic foil capacitor; they were extremely common back in the day.
From my RAF career servicing old aircraft equipment of the same era, i.e. 1950s, I know that those are very likely to degrage and basically 'explode' if power is applied after a long period, messing up the inside of your clock with 'sticky gunk'.
We literally used to mark the packaging of equipment containing such capacitors which was on the shelf in Stores with the date it was last powered up and if 6 months had elapsed it had to be brought back to the Maintenance Bay to have power applied for an hour to ensure the capacitors were 'reformed'.
It is possible to reform old capacitors, but it has to be done carefully.

You can read about reforming such capacitors here: https://www.6v6.co.uk/vcomp/tech_tips/reform_caps.htm

Perhaps take a look inside it before powering it up to see if it has a capacitor. Or sell it and let someone else have responsibility.......

[svenedin]

Interesting what was said about the capacitor. Old Kenwood Chef mixers had a notorious exploding electrolytic capacitor. It’s usually all that’s wrong with them. Just a case of replacing the “banger” which is what I did with my mother's late 1960's Kenwood.

[Myrtles Man]

On the subject of capacitors, this Youtube clip https://www.youtube.com/watch?v=vNiztRfaNzY&t=935s is very instructive in regard to their construction and self-repair capabilities (specifically from about 17 minutes onwards but the whole thing is well worth watching and, for electrical numpties like myself, extremely enlightening).

[geoberni]

On the subject of capacitors, this Youtube clip https://www.youtube.com/watch?v=vNiztRfaNzY&t=935s is very instructive in regard to their construction and self-repair capabilities (specifically from about 17 minutes onwards but the whole thing is well worth watching and, for electrical numpties like myself, extremely enlightening).

The vintage type we're dealing with in the case of the clock, if indeed it has one, is 2 layers of conductive film separated by a waxed paper. This is a good video to explain what goes wrong.
He even deliberately causes a modern tin can type to rupture, which has a deliberate weak spot in it.
https://youtu.be/fj2iGvnznbE